I've got a 05 3500.
The heater control is not working. is there a cable behind the control knob/ If so how easy os it to get to. I get heat out of the center vents but not the floor or defroster, no matter how I
turn the controller.
Thanks,
Matt Murphy

There are cables that connect at the back of the ATC (which is the entire controller - all three knobs together). Check on the forum here for instruction on how to remove it. Just require removing a few Torx screws and some center dash pieces, fairly easy. The cable may have come off. You will just tip that control out, you can't pull it out the whole way because of the cables attached at the back. See what the status is, If one is broken, then you have a lot more work. In any case, make sure you lube them with some decent lubricant like 3 in 1 oil or such. Mine were quite stiff initially, I had checked them because my AC was not working and lubed them then. They work very smoothly now.

You might also check your cabin filter to see if its clogged up. When I bought my Sprinter I had similar problems. Changing the cabin filter allowed enough airflow to allow air to flow on the windshield and be directed to the floor.

I also ended up lubing the pull cables on the center vent control (you can get to them if the radio is out, otherwise, there is a lot to take apart on the console to get to them). However, I had one of the pull cables flexing under compression - I added a small spring to help pull it back (the console has to come apart to add it).

I agree that the defroster mode is not very aggressive. When its real cold and snowy out, I find I go thru a lot of windshield washer fluid to keep the windshield clear. Ice on the windshield is also solved in that way.

When I could only get warm air thru the dash vents, it was solved by spraying graphite lub on and in the pull cables behind the air/heat/vent control unit (the one with the knob you are trying to turn to get the air directed to the floor and/or windshield). This is a common issue.

You might consider replacing the light bulbs in that control unit if you have it apart to lubricate.

Concur, the defrost of the windshield is not very effective. In the manual, they state you need to run the fan on 4 to get it to work. It does not seem to deliver that much air volume to the windshield, but on 4 with the temp on complete hot, it does do the job. But, you wait a long time if you don't manually scrape the outside in the morning!

Had a similar problem on my neglected 2005. The cable was kinked and needed lube. Not to difficult (I did it, and I am not a mechanic) to pull the unit far enough out to straighten or reattach, then lube.